Mechanism for bi-directionally driving a print head

ABSTRACT

This invention relates to an improved mechanism for driving the print head of a printer and is particularly adapted for bidirectionally driving the print head of a wire matrix printer. The mechanism includes a bi-directional motor, preferably a DC motor, which is coupled to the print head. When a print operation is to be performed, the motor is started running in a first direction, causing the print head to move across a print line in a first direction. A first detector is provided for detecting when the print head reaches a predetermined point of a print line when being driven in the first direction, the motor being energized in response to an output from the first detector for reversing its direction of energization for a period of time at least sufficient to overcome the inertia of the print head in the first direction and for substantially stopping the print head. For the preferred embodiment, the motor continues running in its reverse direction to drive the head in its opposite direction, with a second detector being provided to detect the print head reaching a predetermined point on the print line when being driven in the opposite direction. The motor is energized in response to an output from the second detector to again run in the first direction for a period of time at least sufficient to substantially stop the print head.

United States Patent 1 Sloan et al.

[ MECHANISM FOR BI-DIRECTIONALLY DRIVING A PRINT HEAD [75] Inventors: Charles T. Sloan, Cheshire; Nicholas V. Zaccagnino, Stamford, both of Conn.

[73] Assignee: Bunker Ramo Corporation, Oak

Brook, Ill.

[22] Filed: Aug. 6, 1973 [21] Appl. No.: 386,077

[52] US. Cl..... 197/66; 197/1 R; 197/82 [51] Int. Cl B4lj 19/70 [58] Field of Search 197/1, 6.7, 60, 65, 66, 197/19, 64, 82, 93; 101/93 C [56] References Cited UNITED STATES PATENTS 2,797,789 7/1957 Yaeger 197/66 3,167,166 l/1965 Schiebeler.. 197/1 R 3,292,530 12/1966 Martin 197/] R X 3,578,129 5/1971 Kato 197/66 X 3,618,514 11/1971 Nyman et al 197/1 RX 3,638,197 1/1972 Brennan et al.,.v 197/1 R X 3,670,861 6/1972 Zenner et al 197/1 R X 3,752,288 8/1973 Detig et al. 197/1 R 3,787,884 1/1974 Demer 197/1 R X Primary ExaminerEdgar S. Burr Assistant E.\'aminerR. T. Rader Attorney, Agent, or Firm-F, M. Arbuckle May 13, 1975 [57] ABSTRACT This invention relates to an improved mechanism for driving the print head of a printer and is particularly adapted for bidirectionally driving the print head of a wire matrix printer. The mechanism includes a bidirectional motor, preferably a DC motor, which is coupled to the print head. When a print operation is to be performed, the motor is started running in a first direction, causing 'the print head to move across a print line in a first direction. A first detector is provided for detecting when the print head reaches a predetermined point of a print line when being driven in the first direction, the motor being energized in response to an output from the first detector for reversing its direction of energization for a period of time at least sufficient to overcome the inertia of the print head in the first direction and for substantially stopping the print head. For the preferred embodiment, the motor continues running in its reverse direction to drive the head in its opposite direction, with a second detector being provided to detect the print head reaching a predetermined point on the print line when being driven in the opposite direction. The motor is energized in response to an output from the second detector to again run in the first direction for a period of time at least sufficient to substantially stop the print head.

21 Claims, 4 Drawing Figures PATENTEB HAY I 31975 SHEET 2 BF 2 MECHANISM FOR BI-DIRECTIONALLY DRIVING A PRINT HEAD This invention relates to an improved mechanism for driving the print head of a printer and more particularly to a mechanism for bi-directionally driving the print head of a wire matrix printer.

BACKGROUND OF THE INVENTION In most high speed printers, the document carriage or platen remains stationary and the print head is moved across a line of the document to print characters thereon. In earlier printers, the head was incremented for each character position (or for each stroke with matrix print heads printing only a stroke at a time). However, as speeds increased, it was found necessary to store an entire line of characters in a suitable buffer and then print the line on the fly as the print head moved continuously from one end of the print line to the other.

Existing printers for printing on the fly have, in order to achieve substantially uniform characterspacing and quality, attempted to maintain substantially uniform head velocity across the entire print line. In order to achieve the high acceleration and deceleration required for this mode of operation, the motors utilized in these printers have been run continuously in one direction. A first clutch is utilized to connect the print head drive to the motor to move the print head across thee carriage when a line is to be printed, with the clutch disengaging when the end of the line is reached and a brake utilized to quickly stop the print head. A dash-pot may also be used in addition to the brake to stop the head within the required short distance. To return the head from the end-of-line to the beginning-ofline position, either a second clutch may be utilized which connects the continuously running motor in the opposite direction to the head drive mechanism, the clutch disengaging and the head being braked when the start-of-line position is reached; or a spring or similar resilient member may be stretched when the head is driven across the print line, the spring being operative to return the head to the start-of-line position, possibly with the assistance of a braking element, when the clutch disengages.

While the print head drive mechanisms described above are operative to move the head across a print line at substantially uniform speed, they also have a number of disadvantageous features. First, the modes of operations described are inefficient in their use of energy. In a typical application, the printer might have a duty cycle of, for example,.four per cent (4%) (i.e., the print head is actually being moved only 4% of the time the motor is running). Thus, roughly 96% of the energy applied to run the motor is being wasted. In addition to wasting the energy, this unnecessary running of the motor also causes wear on the motor and generates substantial heat, both of which reduce the life of the motor and other related mechanical and electrical elements and significantly increase the maintenance required on the printer. The head generated by the motor must also be dissipated, necessitating additional parts for this function. The motor also generates a fair amount of noise when it is running. and the clutches and brakes are also noisy elements. Since the motor is continuously running in existing printers of the type indicated above, the noise level from these printers is rel- 2 atively high, requiring, in some applications, that acoustic material be added to the printer to reduce the noise to an acceptable level.

It is therefore apparent that an improved drive mechanism for a print head is required which mechanism requires that the motor be driven only when a print operation is being performed and which does not require the use of clutches, brakes, dashpots, and similar elements.

SUMMARY OF THE INVENTION In accordance with the above, this invention provides a mechanism for use in a line printer adapted for printing on the fly, which mechanism drives a print head across a print line. For the preferred embodiment, the mechanism drives the print head in both directions across the line, a print operation being performed when the print head is being driven in at least one of the directions. The mechanism includes a bi-directional motor, preferably a DC motor, and a means for coupling the motor to the print head. The means for coupling is operative to drive continuously the print head in a first direction when the motor is running continuously in one direction and is operative to drive the print head in the opposite direction when the motor is reversed. The mechanism also includes a means operative when a print operation is to be performed for starting the motor running in the first direction, a means for detecting the print head reaching a predetermined point of a print line when being driven in the one direction, and means oprated responsive to the operation of the means for detecting the print head reaching a first predetermined point for energizing the motor to reverse its direction for a period of time at least sufficient to overcome the inertia of the print head in the first direction and for substantially stopping the print head. For the preferred embodiment, the motor continues running in its reverse direction to drive the head in its opposite direction, with a second means for detecting being provided to detect the print head reaching a predetermined point on a print line when a head is being driven in the opposite direction. There is also a means operated responsive to the operation of the second means for detecting the print head reaching the other predetermined point for energizing the motor to again run in the one direction for a period of time sufficient to substantially stop the print head.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings.

BRIEF DESCRIPTIONS OF DRAWINGS FIG. 1 is a perspective view of a printer in which the teachings of this invention may be utilized.

FIG. 2 is a perspective view of a step-down gear mechanism suitable for use in the printer shown in FIG. 1.

FIG. 3 is a schematic block diagram of a circuit adapted for controlling energization of the motor shown in FIG. 1 in accordance with the teachings of this invention.

FIG. 4 is a diagram illustrating the velocity of the print head at various character positions of a print line.

DETAILED DESCRIPTION Referring now to FIG. 1, it is seen that the printer includes a housing with a shelf plate 12. A platen 14 supporting a document 16 on which printing is to be performed is supported in housing 10. In front of platen 14 is a rail 18 and there is also a rail 20 mounted on plate 12. A print head assembly 22 is journaled to move along rails 18 and 20. Print head assembly 22 includes a print head 24 a print head support 26 and a pair of shutters 28L and 28R projecting from holder 26. The function of the shutters 28 will be described later. A bumper 30 of a resilient material such as rubber is mounted at each end of rail 18.

A toothed belt 32 of rubber or similar material passes through and is secured to print head support 26. Belt 32 passes over an idler roller 34 at the left end of housing 10 and over a drive roller 36 at the right end of the housing. Roller 36 is connected through a shaft 38 and a gear box 40 to a motor 42. For a preferred embodiment of the invention, motor 42 is a brushless DC motor which runs at a multiple (for example 6) of the rate at which roller 36 is rotated in order to move assembly 22 from one end of rails 18 and 20 to the other (i.e., across a print line).

FIG. 2 illustrates a gear mechanism which may be employed in gear box 40 for a preferred embodiment of the invention. Referring to FIG. 2, shaft 44 from motor 42 has a worm 46 mounted on its end. Worm 46 engages a worm gear 48 which is mounted on the end of shaft 38. The relative sizes of worm 46 and worm gear 48 are such that the desired step down in the speed of rotation of shaft 38 relative to the speed of rotation of shaft 44 is achieved.

Mounted near each end of rail 20 is a detector housing 50, each of the housings containing a light emitter 52 and a light detector (for example, a photocell) 54. A set screw 56 is provided on each housing 50 so as to permit the housing 50 to be moved adjacent to a selected character position along the print line: The relative positions of housing 50 and shutters 28 are such that, when assembly 22 moves adjacent a housing 50, the corresponding shutter 28 moves between the light emitter 52 and photocell 54 of the housing, causing a predetermined change in the photocell output.

A synchronization track 58 is also mounted on plate 12, track 58 having a sync marking 60 for each stroke position at which printing may occur. Assembly 22 includes an inverted U-shaped member 62 having an arm on each side of track 58. In one arm of housing 62 is mounted a light source and in the other arm a photocell. The manner in which the sync track is utilized to control printing, and other aspects of the actual printing operation do not form part of the present invention and will therefore not be described further herein. A more detailed description of the operation of a printer of the general type shown in FIG. 1 may be found in U.S. Pat. No. 3,703,949 entitled High Speed Printer, issued Nov. 28, 1972 to Robert Howard, et al.

OPERATION FIG. 3 illustrates an electrical control circuit designed to cause the printer of FIG. 1 to operate in accordance with the teachings of this invention. When a print operation is to begin, assembly 22 should be positioned at the left end of rails 18 and 20 (i.e., at the beginning of a print line). With assembly 22 in this position, left shutter 28L is positioned adjacent to detector housing 50L causing photocell 54L to generate an output on line (this output may actually be a ground level). The signal on line 70 is applied asone input to AND gate 72. At the start of the print operation, a print signal appears on print line 74. If there is a signal on line 70 at this time, indicating that print head assembly 22 is properly positioned at the left margin, AND gate 72 is fully conditioned to generate an output on line 76 which is applied to set flip flop 78 to its set or forward condition. The print signal on line 74 is also applied to set enable flip flop 80. The output signals on line 82 from the forward side of flip flop 78 and on enable line 84 from the set side of enable flip flop are applied to fully condition AND gate 86. The resulting output signal on line 88 is applied through OR gate 90 to enable switch 92, permitting positive voltage from source 94 to be applied through line 96 and the forward side of push-pull amplifier 98 to energize the coils of brushless DC motor 42 in a sequence such as to cause the motor to rotate in a forward direction. For purposes of this discussion, the forward direction will be considered to be the direction in which motor 42 is rotated in order to cause print head assembly 22 to move from left to right along rails 18 and 20. The speed at which motor 42 rotates is controlled by summing amplifier 100 which compares the current from a constant current source 102 with the current generated from the motor back EMF. This current on line 104 is rectified by back EMF detector 106 and fed through speed control variable resistor 108. Output line 112 from summing amplifier 100 is applied to control the stator field of motor 98 and thus the speed at which the motor rotates. The time required for motor 98 to reach its full operating speed is, referring to FIG. 4, roughly equal to 300 milliseconds for a preferred embodiment of the invention. The acceleration time is thus equal to roughly 20 percent of the time required for the print head to travel across a print line (i.e., 1.5 seconds). Roughly 25 characters would be printed during this acceleration time of assembly 22.

Ordinarily, the time period which passes before the printer reaches full velocity would not be acceptable because of the poor print quality of the characters printed in the first few character positions. However, as is described in greater detail in the beforementioned Howard, et al., patent, sync track 58 assures that printing occurs at the same stroke position on the print line regardless of the speed at which the print head is moving (i.e., a print stroke occurs when a sync marking 60 is detected and the timing of the print stroke is thus totally independent of the speed at which the head moves). Thus, instantaneous acceleration of the print head assembly 22 to its maximum velocity is not required.

When head assembly 22 approaches the right side of the print line, shutter 28R moves between light source 52R and light detector 54R of detector assembly 50R causing a predetermined output from photodetector 54R on line 114. This signal is applied to reset flip flop 78 to its reversed state. Flip flop 78 being reset to its reversed state terminates the signal on forward line 82, deconditioning AND gate 86, and causes a signal to appear on reverse line 116. The deconditioning of AND gate 86 terminates the signal on line 88 resulting in a signal being applied to one-shot 118 causing the oneshot to generate an output on line 120. The duration of the signal on line 120 depends on the setting of variable resistor 122 and might, for a typical application, be roughly 50 milliseconds. The signal on reverse line 116, in conjunction with the signal on enable line 84, fully conditions AND gate 122 to generate an output on line 124. The signals on line 120 and 124 are applied as inputs to OR gate 126, the output from which is applied to condition switch 128 to pass the positive potential level from source 94 through the reverse side of pushpull amplifier 98 to the stator windings of motor 42. Motor 42 being energized from the reverse side of push-pull amplifier 98 causes the phase of the current applied to the coils of the motor stator to be changed so as to cause the motor to rotate in the reverse direction. This reversal of phase in the stator windings tends to oppose the continued rotation of motor 42 in the forward direction, effectively braking the motor and thus the print head assembly driven thereby. By proper selection of the position for detector 50R, assembly 22 may be caused to stop, under the influence of the reverse energization of motor 42, at or just short of the right end of the print line. For the preferred embodiment of the invention, detector 50R would be energized roughly 50 milliseconds before the end of the print line in order to achieve this objective. Bumper 30R assures that the print assembly is in fact stopped at the right end of the print line and absorbs any slight momentum which the head assembly may still have when reaching this point.

At roughly the point where the inertia in the forward direction of the motor, gears, rollers, belt, assembly 22, and other moving elements has been overcome, and the head assembly substantially stopped at the right margin, one-shot 118 times out removing the input on line 120 to OR gate 126. However, since AND gate 122 is still fully conditioned at this time, OR gate 126 continues to condition switch 128, resulting in the continued energization of motor 42 in the reverse direction. This causes the motor to reverse its direction of rotation resulting in head assembly 22 being moved from right to left across the print line.

As the assembly approaches the left margin. shutter 28L passes between the components of detector 50L, causing an output on line 70 from detector 54L. The signal on line 70 is applied as one input to AND gate 130, the other input to this AND gate being reverse line 116, and the output from inverter 131, the input to which is print line 74. AND gate 130 is thus fully conditioned at this time to generate an output on line 132 which is applied through OR gate 134 to reset enable flip flop 80. The resetting of enable flip flop 80 removes the signal from line 84, thus deconditioning AND gate 122. The resulting loss of signal on line 124 deconditions switch 128, preventing signal from being applied to the reverse side of push-pull amplifier 98. In addition, the loss of signal on line 124 also turns on oneshot circuit 136 for a period of time determined by the setting of variable resistor 138. Again, for the preferred embodiment of the invention, this period of time may be considered to be roughly 50 milliseconds. The output from one-shot 136 on line 140 is applied through OR gate 90 to condition switch 92, permitting a positive voltage to be applied to the forward side of pushpull amplifier 98. The 50 millisecond duration of oneshot 136 permits motor 42 to be forward energized for a period of time sufficient to substantially overcome the inertia in the reverse direction of the motor, head assembly, and other moving components, and thus to substantially stop the head assembly at its left margin. The motor is not, however, energized in a forward direction long enough for the head to start moving in the forward direction. Any residual momentum of head assembly 22 in the reverse direction is absorbed by bumper 30L, permitting head assembly 22 to be smoothly and quietly brought to a stop at the desired left margin position without the use of noisy and expensive brakes and/or dashpots.

In the discussion above, it was assumed that head assembly 22 was initially positioned at the left margin when a print signal was received on line 74. While this would normally be the case, it is possible that, as a result of an incompleted operation on a previous cycle, the operator intentionally or unintentionally moving the head assembly, or some other reason, that the head would not be positioned at the left margin when the print signal is received. Under this condition, enable flip flop would still be set, causing a signal to appear on line 84; however, AND gate 72 would not be fully conditioned resulting in flip flop 78 remaining in its reverse condition. Signals appearing on line 84 and 116 fully condition AND gate 122 to generate an output on line 124 which is applied to condition switch 128 resulting in the reverse energization of motor 42 in the manner previously indicated. Motor 42 being reversed energized causes head assembly 22 to be moved toward the left margin. When the head approaches the left margin, shutter 28L passes between the elements of detector 50L causing a signal to appear on line 70. The duration of the print signal on line 74 is such that the signal still appears on this line when the head reaches the left margin resulting in AND gate 72 being fully conditioned to transfer flip flop 78 to its forward state. It is noted that the continued presence of the signal on print line 74 prevents AND gate 130 from being fully conditioned, leaving enable flip flop 80 set. The setting of flip flop 78 to its forward state and the termination of the signal on line 124 from AND gate 122 result in signals on both lines 88 and 140, causing switch 92 to be conditioned to energize motor 42 in the forward direction in a manner previously indicated. This forward energization of the motor is effective to terminate the reverse direction movement of assembly 22 at or near the left end of its travel path and to then cause the assembly to move in the forward direction with a velocity profile as shown in FIG. 4, the printing ofline occurring during the forward movement of the assembly.

In the discussion above it has also been assumed that only one line is being printed at a time, the printer being stopped at the start-of-line position after each line is printed until a new print signal is received on line 74. However, in some applications, a buffer containing several lines of characters may be utilized. ln such applications, a signal would remain on print line 74 so long as there is a line of characters waiting to be printed. With a signal on line 74, the resetting of enable flip flop 80 when assembly 22 reaches the left margin is inhibited and flip flop 78 is set to its forward condition. Thus, the printing of successive lines continues until all lines have been printed and the signal on line 74 terminates.

It is also possible, in some applications, that the printing speed could be increased by printing both on the forward (i.e., left to right) movement of the print head assembly across a print line and on the reverse (i.e.,

right to left) movement of the head assembly. For a printer operating in this mode, an additional input to OR gate 134 would be provided, this input being the output from an AND gate, the inputs to which are line 114 and forward line 82. Thus, enable flip flop 80 would be reset when the right margin was reached as well as when the left margin was reached. One-shot 118 reverse energizes motor 42 for a sufficient period of time to assure that the head assembly is stopped at the right margin. For operating in this mode, it might also be desired to reset flip flop 78 to its reverse state in response to an output from an AND gate the inputs to which are lines 114 and print line 74.

A brushless DC motor has been indicated above for the motor 42 because it generates less radio frequency interference than standard DC motors. However, in applications where RF interference is not a problem, a standard DC motor may be utilized. Because of its superior braking characteristics, a stepping motor may also be utilized. One advantage of using a stepping motor is that it has more low speed torque than a standard DC motor and is therefore capable of developing the required torque without running at a speed several times greater than that required for the rotation of drive roller 36. Thus, with a stepping motor, the motor could be connected directly to roller shaft 38, eliminating the need for gear box 40. However, a stepping DC motor utilizes significantly more energy than a brushless DC or standard DC motor, having hold currents continuously applied to it. While an AC motor might also be utilized, a DC motor is preferred because it is easier to control.

A print head drive mechanism has thus been provided which requires that current'be applied to the drive motor 42 only during the brief time intervals when the print head assembly is actually being moved. Thus, for the embodiment of the invention shown in the figures, the motor would be energized for only three seconds per print line. In a typical application this represents a 4%. duty cycle. Braking of the pring head assembly is achieved by reverse energization of the drive motor, eliminating the need for brakes and dash-pots. Thus, a print head drive assembly is provided which is substantially less expensive to build, operate, and maintain. The drive assembly also runs significantly quieter than existing print head drives and, because it has fewer parts and a significantly shorter duty cycle, it is significantly more reliable.

While the invention has been particularly shown and described above with reference to a preferred embodiment thereof, the foregoing and other changes in form and detail may be made therein while still remaining within the spirit and scope of the invention.

What is claimed is:

l. ln a line printer adapted for printing a material on the fly, a mechanism for bi-directionally driving a print head across a print line with the print head being operated to provide a plurality of successive print strokes in a print line when the print head is driven in at least a first direction, said mechanism comprising:

a bi-directional motor adapted to be continuously energized in one direction for continuously running in one direction and continuously energized in another direction for continuously running in another direction;

means for coupling said motor to said print head, said means for coupling being operative to continuously drive said print head in said first direction when said motor is running in one direction and being operative to continuously drive the print head in the opposite direction when the motor is running in the other direction;

means movedwith said print head for controlling each print stroke in accordance with the print head position to apply print to said material while said print head is driven in said first direction; means operative when a print operation is to be performed for starting said motor running continuously in said one direction to continuously drive said print head in said first direction with said print head operated to provide a plurality of successive print strokes; first detecting means operated while said print head is driven in said first direction and after said print head is operated to provide a plurality of successive print strokes in a print line for detecting said print head reaching a predetermined point of a print line when said print head is driven in said first direction;

means operated responsive to the operation of said first detecting means for continuously energizing said motor in said other direction to brake said motor and then continuously run said motor in said other direction, while the inertia of the print head carryies said print head a distance in said first direction past said predetermined point before said print head stops and is thereafter continuously driven in said opposite direction by said motor energized in said other direction;

second detecting means operated for detecting said print head reaching another predetermined point on a print line when said print head is driven in said opposite direction; and

means operated responsive to the operation of said second detecting means for energizing said motor to again run in said one direction for a period of time sufficient to substantially stop said print head.

2. A mechanism as claimed in claim 1 wherein said motor is a DC motor.

3. A mechanism as claimed in claim 2 wherein said motor is a brushless DC motor.

4. A mechanism as claimed in claim 2 wherein said motor is a stepping motor.

5. A mechanism as claimed in claim 1 wherein said coupling means includes a toothed belt to which said print head is attached, a drive roller for said belt, and means connecting said motor to said drive roller.

6. A mechanism as claimed in claim 5 wherein said motor rotates at a speed substantially greater than that for said roller; and

wherein said connecting means is a gear means operative to step down the speed of said motor to that required by said roller.

7. A mechanism as claimed in claim 5 wherein said gear means includes a worm connected to said motor and a worm gear connected to said roller.

8. A mechanism as claimed in claim 1 including a resilient bumper positioned at either end of the travel path of said print head.

9. A mechanism as claimed in claim 1 including means operative when a print operation is to be performed for detecting and indicating if the print head is at a selected end of a print line; and

means responsive to said detecting and indicating means indicating that the print head is not at the selected end of the print line for energizing said motor in the reverse direction, said means for starting said motor running in said first direction being operative in response to said detecting and indicating means indicating that the print head is at the selected end of the print line. 10. A mechanism as claimed in claim 1 wherein said first and second means for detecting include, for each of said means, a shutter mounted to move with said print head, and a detector mounted adjacent the predetermined point of the print line, said detector having a light source normally directing light at a photodetector, said light source and photodetector being spaced and positioned such that the shutter passes between the light source and photocell of the corresponding detector when the print head is adjacent the predetermined point.

11. In a line printer adapted for printing on the fly, a mechanism for bi-directionally driving a print head across a print line, a print operation being performed when the print head is being driven in at least one of said directions, said mechanism comprising:

a bi-directional motor; means for coupling said motor to said print head, said means for coupling being operative to drive said print head in a first direction when said motor is running in one direction and being operative to drive the print head in the opposite direction when the motor is reversed; an enable flip flop; a forward-reverse flip flop; means operative when a print operation is to be performed for setting the enabled flip flop and for setting said forward-reverse flip flop to its forward state; means responsive to said enable flip flop being set and to said forward-reverse flip flop being in its for ward state for energizing said motor to run in said one direction; first means for detecting said print head reaching a predetermined point of a print line when being driven in said first direction; means responsive to said first means for detecting for resetting said forward-reverse flip flop to the reverse state; means responsive to said enable flip flop being set and to said forward-reverse flip flop being in its reverse state for energizing said motor to reverse its direction, the inertia of the print head carrying it a distance past said predetermined point before it stops and starts moving in said opposite direction; second means for detectingsaid print head reaching a predetermined point on a print line when being driven in said opposite direction; means responsive to said second means for detecting or resetting said enable flip flop; and means operative when said motor is being driven in said opposite direction and said enable flip flop is reset for energizing said motor in said one direction for a short period of time which is sufficient to substantially stop said print head but not sufficient to start said print head moving in said first direction. 12. A mechanism as claimed in claim 11 including means operative in the absence of an output from said second means for detecting for inhibiting the setting of said forward-reverse flip flop to its forward condition when a print operation is to be performed, said enable flip flop being set to, in conjunction with said forwardreverse flip flop remaining in its reverse condition, energized said motor to move the print head in said opposite direction until an output is obtained from said second means for detecting.

13. A mechanism as claimed in claim 11 including means operative each time said enable flip flop is reset or the state of said forward-reverse flip flop is changed with said enable flip flop set for energizing said motor in a direction opposite from the direction in which it is running for a period of time which is sufficient to substantially stop said print head but not sufficient to cause said print head to start moving in the direction opposite from the direction in which it was running.

14. A mechanism as claimed in claim 13 wherein said means for energizing the motor in the direction opposite of that in which it was running includes a pair of one-shot circuits, each of said one-shot circuits being connected to be turned on when the signal causing the motor to be energized in a given direction terminates, and being operative to cause the motor to be energized to rotate in a direction opposite from the given direction.

15. In a line printer adapted for printing a material on the fly, a mechanism for driving a print head in a first direction with said print head being operated to provide a plurality of successive print strokes across a print line, comprising:

a bi-directional motor adapted to be continuously energized in one direction for continuously running in one direction and continuously energized in another direction for continuously running in another direction;

means for coupling said motor to said print head, said means for coupling being operative to continuously drive said print head in a first direction when said motor is running in one direction and being operative to continuously drive the print head in the opposite direction when the motor is running in the other direction;

means moved with said print head for controlling each print stroke in accordance with the print head position to apply print to said material while said print head is driven in said first direction;

means operative when a print operation is to be performed for starting said motor running continuously in said one direction to continuously drive said print head in said first direction with said print head operated to provide a plurality of successive print strokes;

detecting means operated while said print head is driven in said first direction and after said print head is operated to provide a plurality of successive print strokes in a print line for detecting said print head reaching a predetermined point of a print line; and

means operated responsive to the operation of said detecting means for continuously energizing said motor in said other direction to brake said motor while said print head is carried in said first direction for a period of time at least sufficient to overcome the inertia of the print head in said first direction and to substantially stop the print head. 16. A mechanism as claimed in claim 15 wherein said motor is a DC motor.

17. A mechanism as claimed in claim 15 wherein said motor is a brushless DC motor.

18. A mechanism as claimed in claim wherein said motor is a stepping motor.

19. A mechanism as claimed in claim 15 wherein said coupling means includes a toothed belt to which said print head is attached, a drive roller for said belt, and means connecting said motor to said drive roller.

20. A mechanism as claimed in claim 15 wherein said motor rotates at a speed substantially greater than that for said roller; and

wherein said connecting means is a gear means operative to step down the speed of said motor to that required by said roller.

21. A mechanism as claimed in claim 15 including means operative when a print operation is to be performed for detecting and indicating if the print head is at a selected end of a print line; and

means responsive to said detecting and indicating means indicating that the print head is not at the selected end of the print line for energizing said motor in the reverse direction, said means for starting said motor running in said first direction being operative in response to said detecting and indicating means indicating that the print head is at the selected end of the print line. 

1. In a line printer adapted for printing a material on the fly, a mechanism for bi-directionally driving a print head across a print line with the print head being operated to provide a plurality of successive print strokes in a print line when the print head is driven in at least a first direction, said mechanism comprising: a bi-directional motor adapted to be continuously energized in one direction for continuously running in one direction and continuously energized in another direction for continuously running in another direction; means for coupling said motor to said print head, said means for coupling being operative to continuously drive said print head in said first direction when said motor is running in one direction and being operative to continuously drive the print head in the opposite direction when the motor is running in the other direction; means moved with said print head for controlling each print stroke in accordance with the print head position to apply print to said material while said print head is driven in said first direction; means operative when a print operation is to be performed for starting said motor running continuously in said one direction to continuously drive said print head in said first direction with said print head operated to provide a plurality of successive print strokes; first detecting means operated while said print head is driven in said first direction and after said print head is operated to provide a plurality of successive print strokes in a print line for detecting said print head reaching a predetermined point of a print line when said print head is driven in said first direction; means operated responsive to the operation of said first detecting means for continuously energizing said motor in said other direction to brake said motor and then continuously run said motor in said other direction, while the inertia of the print head carryies said print head a distance in said first direction past said predetermined point before said print head stops and is thereafter continuously driven in said opposite direction by said motor energized in said other direction; second detecting means operated for detecting said print heAd reaching another predetermined point on a print line when said print head is driven in said opposite direction; and means operated responsive to the operation of said second detecting means for energizing said motor to again run in said one direction for a period of time sufficient to substantially stop said print head.
 2. A mechanism as claimed in claim 1 wherein said motor is a DC motor.
 3. A mechanism as claimed in claim 2 wherein said motor is a brushless DC motor.
 4. A mechanism as claimed in claim 2 wherein said motor is a stepping motor.
 5. A mechanism as claimed in claim 1 wherein said coupling means includes a toothed belt to which said print head is attached, a drive roller for said belt, and means connecting said motor to said drive roller.
 6. A mechanism as claimed in claim 5 wherein said motor rotates at a speed substantially greater than that for said roller; and wherein said connecting means is a gear means operative to step down the speed of said motor to that required by said roller.
 7. A mechanism as claimed in claim 5 wherein said gear means includes a worm connected to said motor and a worm gear connected to said roller.
 8. A mechanism as claimed in claim 1 including a resilient bumper positioned at either end of the travel path of said print head.
 9. A mechanism as claimed in claim 1 including means operative when a print operation is to be performed for detecting and indicating if the print head is at a selected end of a print line; and means responsive to said detecting and indicating means indicating that the print head is not at the selected end of the print line for energizing said motor in the reverse direction, said means for starting said motor running in said first direction being operative in response to said detecting and indicating means indicating that the print head is at the selected end of the print line.
 10. A mechanism as claimed in claim 1 wherein said first and second means for detecting include, for each of said means, a shutter mounted to move with said print head, and a detector mounted adjacent the predetermined point of the print line, said detector having a light source normally directing light at a photodetector, said light source and photodetector being spaced and positioned such that the shutter passes between the light source and photocell of the corresponding detector when the print head is adjacent the predetermined point.
 11. In a line printer adapted for printing on the fly, a mechanism for bi-directionally driving a print head across a print line, a print operation being performed when the print head is being driven in at least one of said directions, said mechanism comprising: a bi-directional motor; means for coupling said motor to said print head, said means for coupling being operative to drive said print head in a first direction when said motor is running in one direction and being operative to drive the print head in the opposite direction when the motor is reversed; an enable flip flop; a forward-reverse flip flop; means operative when a print operation is to be performed for setting the enabled flip flop and for setting said forward-reverse flip flop to its forward state; means responsive to said enable flip flop being set and to said forward-reverse flip flop being in its forward state for energizing said motor to run in said one direction; first means for detecting said print head reaching a predetermined point of a print line when being driven in said first direction; means responsive to said first means for detecting for resetting said forward-reverse flip flop to the reverse state; means responsive to said enable flip flop being set and to said forward-reverse flip flop being in its reverse state for energizing said motor to reverse its direction, the inertia of the print head carrying it a distance past said predetermined point before it stops and starts moving in said opposite direction; second meAns for detecting said print head reaching a predetermined point on a print line when being driven in said opposite direction; means responsive to said second means for detecting or resetting said enable flip flop; and means operative when said motor is being driven in said opposite direction and said enable flip flop is reset for energizing said motor in said one direction for a short period of time which is sufficient to substantially stop said print head but not sufficient to start said print head moving in said first direction.
 12. A mechanism as claimed in claim 11 including means operative in the absence of an output from said second means for detecting for inhibiting the setting of said forward-reverse flip flop to its forward condition when a print operation is to be performed, said enable flip flop being set to, in conjunction with said forward-reverse flip flop remaining in its reverse condition, energized said motor to move the print head in said opposite direction until an output is obtained from said second means for detecting.
 13. A mechanism as claimed in claim 11 including means operative each time said enable flip flop is reset or the state of said forward-reverse flip flop is changed with said enable flip flop set for energizing said motor in a direction opposite from the direction in which it is running for a period of time which is sufficient to substantially stop said print head but not sufficient to cause said print head to start moving in the direction opposite from the direction in which it was running.
 14. A mechanism as claimed in claim 13 wherein said means for energizing the motor in the direction opposite of that in which it was running includes a pair of one-shot circuits, each of said one-shot circuits being connected to be turned on when the signal causing the motor to be energized in a given direction terminates, and being operative to cause the motor to be energized to rotate in a direction opposite from the given direction.
 15. In a line printer adapted for printing a material on the fly, a mechanism for driving a print head in a first direction with said print head being operated to provide a plurality of successive print strokes across a print line, comprising: a bi-directional motor adapted to be continuously energized in one direction for continuously running in one direction and continuously energized in another direction for continuously running in another direction; means for coupling said motor to said print head, said means for coupling being operative to continuously drive said print head in a first direction when said motor is running in one direction and being operative to continuously drive the print head in the opposite direction when the motor is running in the other direction; means moved with said print head for controlling each print stroke in accordance with the print head position to apply print to said material while said print head is driven in said first direction; means operative when a print operation is to be performed for starting said motor running continuously in said one direction to continuously drive said print head in said first direction with said print head operated to provide a plurality of successive print strokes; detecting means operated while said print head is driven in said first direction and after said print head is operated to provide a plurality of successive print strokes in a print line for detecting said print head reaching a predetermined point of a print line; and means operated responsive to the operation of said detecting means for continuously energizing said motor in said other direction to brake said motor while said print head is carried in said first direction for a period of time at least sufficient to overcome the inertia of the print head in said first direction and to substantially stop the print head.
 16. A mechanism as claimed in claim 15 wherein said motor is a DC motor.
 17. A mechanism as claimed in claim 15 wherein Said motor is a brushless DC motor.
 18. A mechanism as claimed in claim 15 wherein said motor is a stepping motor.
 19. A mechanism as claimed in claim 15 wherein said coupling means includes a toothed belt to which said print head is attached, a drive roller for said belt, and means connecting said motor to said drive roller.
 20. A mechanism as claimed in claim 15 wherein said motor rotates at a speed substantially greater than that for said roller; and wherein said connecting means is a gear means operative to step down the speed of said motor to that required by said roller.
 21. A mechanism as claimed in claim 15 including means operative when a print operation is to be performed for detecting and indicating if the print head is at a selected end of a print line; and means responsive to said detecting and indicating means indicating that the print head is not at the selected end of the print line for energizing said motor in the reverse direction, said means for starting said motor running in said first direction being operative in response to said detecting and indicating means indicating that the print head is at the selected end of the print line. 